Question

why does a piece of Steel sink in water but floats on Mercury​

Answer 1

because density of iron is less then the density of mercury

Answer 2

$<b>Steel balls sink in water but they don't sink in mercury. ... When the steel balls are placed in mercury then it floats in it because the density of the mercury is greater than the density of the steel balls. So, the steel balls displace the water equal to their weight and hence float.</b>$